Alcohol has been getting a pretty bad health rap in recent years. Recommended weekly maximums have gone down from 28 units for men and 21 for women to 14 for both, which is not much more than a bottle of wine.

Moderate boozers had a rare opportunity to raise a virtuous glass yesterday when research was published suggesting that complete abstinence in middle age was associated with higher risk of dementia in later life. Moderate drinkers were at the lowest risk, with the benefits starting to be cancelled out with each drink above 14 units.

Middle-aged non-drinkers may have 'higher risk' of dementia

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So can we expect official government health advice to start recommending moderate drinking? Of course not. One study into one condition does not cancel out many other studies into the deleterious effects of boozing. But even if the evidence overwhelmingly supported the view that moderate drinking was best for our health, the government would not advise us to do it.

The proof is that this was indeed the case until very recently. For some time, studies repeatedly produced graphs with a J-shaped curve, showing both abstinence and excessive consumption associated with the worst health outcomes, with moderate drinkers enjoying the best health. It was only in January 2016 that the Department of Health revised its guidelines and claimed the best evidence now suggested that there was no “safe” level of alcohol consumption and every glass increased cancer and heart disease risk.

These new guidelines have been contested, but whether they are right or wrong, the point is that for years people who looked at the data repeatedly found the J-shaped curve, but no official source ever recommended the lifestyle it pointed to: one of moderate drinking. Changes in evidence don’t change the general tenor of anti-alcohol advice, they merely change how forcibly the authorities dish it out.

Why should this be so? One reason is that we like to think in clean, clear categories of good and bad. With our puritanical Protestant history, alcohol has always fallen on the dark side of this divide. So when the truth turns out to be complicated, rather than accept this maturely, we refuse to acknowledge the good and carry on as though it were all bad. Because drunkenness is sinful, moral condemnation of it trumps any other redemptive qualities it might have.

If we are to ask where this unhelpful moralising came from, the prime suspect has to be our long tradition of Christianity. As Nietzsche memorably put it, the key divide in Christian morality is between good and evil. For the ancient Greeks, however, it was between good and bad, where “good” was simply whatever promoted human flourishing and “bad” whatever hindered it. This way of thinking is subtly contextual. In Aristotle’s famous example, what would be a good diet for the wrestler Milo would be an excessive one for a sedentary scholar.

There is no moralising here, just a sober assessment of what is beneficial. Christian morality, in contrast, treats everything as though it were inherently good or bad, and once something is labelled wicked, it takes a lot for people to accept it might be good for us after all.

Six-year-old Alfie Dingley with his parents Drew and Hannah Deacon, who have won their fight to let him take medicinal cannabis Photograph: Jack Taylor/Getty Images

Indirect evidence of how powerful this process of moralisation is can be seen in the way authorities have dealt with other illicit drugs. It took the recent scandal in which Billy Caldwell and Alfie Dingley were denied much-needed medicines for the government to finally accept what all qualified experts already knew: that there was a vital role for medicinal cannabis.

The blanket dismissal of all intoxicating drugs has also hampered scientists researching the therapeutic benefits of psychedelics. One such thwarted investigator is David Nutt, who knows a thing or two about institutional stupidity on these matters, having once been the government’s chief drugs adviser. That role required him to take an evidence-based look at policy. But when he did, and concluded that ecstasy and LSD were less dangerous than alcohol, he was sacked.

Aside from the pure moralising, a more understandable but no less erroneous reason for refusing to recommend any consumption of intoxicants is fear of the slippery slope. Even if 21 units of wine a week does turn out to be healthy, 21 bottles of wine is not. Similarly, drug use can slide into drug misuse. Give a green light to moderate drinking, so the fear goes, and heavy use is sure to follow.

This paternalism might be benign in intent but it is malign in effect. If it is to have any credibility, health advice must be consistent with the evidence, not gerrymandered to anticipate its potential misuse. Sadly, however, health advice almost always goes through the distorting filter of officials anticipating behaviour change. Hence the fact that potatoes don’t count as one of your five-a-day: officially, because they are not as nutritious as other vegetables; but really because it was feared people would scoff bags of crisps and chips and think they were being healthy. Similarly, although there is no scientific basis at all for the “10,000 steps a day” exercise advice, it’s recommended as a nice round number which people can easily remember – and feel bad and disillusioned about not achieving.

Health advice too often follows the principle of the noble lie. Rather than being told the plain truth, we are told what the authorities believe will lead us to behave properly, when “properly” means not just in the way that is most prudent for ourselves, but what is seen to be morally appropriate. This means that whatever the truth about healthy drinking or drug-taking is, we can’t trust government health advice to provide it. When the best current scientific evidence meets moralising paternalism, it is truth that starts to bend.